1. Field of the Invention
The invention relates to a method and an apparatus for controlling a brushless DC motor.
2. Description of the Related Art
Brushless DC motors (BLDC motors) are machines with permanent magnets in the rotor. The rotor produces the excitation field. The stator of the BLDC motor has a three-phase winding and is fed by an inverter, which makes available three-phase current. BLDC motors are self-commutated machines and require a rotation angle position encoder for operation. The armature current forms, together with the field of the permanent magnet, a torque which is as constant as possible over time. The reconnection of the stator field takes place with the aid of the inverter as a function of the rotor position so that a constant angle of π/2 electrical is set between the stator rotating magnetomotive force and the rotor field. The rotor position is preferably detected via sensors or derived from terminal voltages and/or terminal currents. The operational response largely corresponds to that of the DC motor. BLDC motors have very good dynamics and can be controlled easily. The brushless technology is free from wear and maintenance.
BLDC motors are used in particular for power-assisted steering systems in motor vehicles. Power-assisted steering systems are devices for increasing the steering force which are designed to increase a steering force applied by the driver of the motor vehicle. The force is increased electromechanically by means of the BLDC motor. The power-assisted steering system preferably comprises a torque sensor, which detects the present torque transmitted by the driver onto the steering wheel. The torque is then increased, it being possible for the gain factor to be dependent on various operational variables of the motor vehicle. Then, the BLDC motor is controlled in a corresponding manner. The BLDC motor is preferably coupled to the steering device of the motor vehicle by means of a transmission. Irregularities in the torque which is emitted by the BLDC motor are perceived by the driver as jolts on the steering wheel. A regularity of the torque is therefore essential for a high degree of driving comfort. Such irregularities of the torque are also referred to as torque ripple.
DE 199 38 678 A1 has disclosed a method and an apparatus for controlling a BLDC motor which is operated with block commutation of the voltage. The DC motor is controlled in such a way that the voltage applied deviates from the voltages provided for the block commutation as a function of the rotation speed of the motor and its current consumption. For this purpose, a table is provided from which altered pulse widths when switching over the windings of the DC motor as a function of the rotation speed N and current values are read and controlled in a corresponding manner.
DE 101 26 470 A1 has disclosed a method for operating a steering device with a device for increasing the steering force. The device for increasing the steering force is an electrically commutated electric motor, to be precise a BLDC motor. The BLDC motor is operated using current control. A voltage to be produced is determined as a function of a desired current, a desired rotation speed and a temperature of the electric motor.